In recent years, semiconductor devices used in laser diodes or light receiving elements have been put to practical use over a wide light wavelength range from near infrared to blue.
Substrates used in these semiconductor devices are usually manufactured by epitaxial growth. It has been well known that heterojunctions for connection of different materials are used in these substrates. Since a laser diode having a double heterojunction structure was proposed by H. Kroemer in 1962, heterojunctions have been practically used in various epitaxial growth techniques, such as, liquid phase epitaxy and vapor phase epitaxy.
In epitaxial growth of a semiconductor layer having a material different from that of a substrate, it is common knowledge to grow a semiconductor material having substantially the same lattice constant as that of the substrate. If a material having a lattice constant different from that of a substrate is used in such epitaxial growth, the difference in the lattice constant accumulates strain energy and then induces misfit dislocations in an epitaxial layer with a thickness exceeding a critical thickness. This forms a surface pattern called a cross hatch pattern. The misfit dislocation induced in the epitaxial layer degrades crystallinity in the epitaxial layer and precludes production of a high-performance device.
Recently, methods for epitaxial growth of a semiconductor layer having a lattice constant different from that of a substrate have been proposed, for example, a method for forming a strained super lattice in which a total amount of strain in an epitaxial layer is restricted below the strain of the critical thickness and a method for growing an epitaxial layer with a thickness exceeding the critical thickness (for example, refer to Patent Document 1).
According to Patent Document 1, in an InGaAsP material system, which is lattice-matched with an InP substrate having a bandgap of 1.35 eV, InGaAs having a bandgap of 0.75 eV can be grown on the InP substrate. In this case, the device can cover the light wavelength ranging from 0.92 to 1.65 μm. Patent Document 1 also discloses that the epitaxial growth can be carried out by metal organic chemical vapor deposition (MOCVD) or chloride vapor phase epitaxy (chloride VPE). In general, MOCVD is advantageous in productivity and a precise control of thickness and composition, so that at present most epitaxial crystal substrates used in semiconductor devices are manufactured by MOCVD.
A near-infrared region of 1.9 to 2.6 μm includes specific absorption bands inherent in water and various gasses. Hence, this region is available in sensors that measure the content of water in various materials from vapor to solid and radiation heat at a temperature in a middle range, for example, of 200 to 500° C. Such sensors are very effective in non-contact monitoring in various heating processes. If such image sensors can be manufactured at high yield using semiconductor devices, the image sensors will be effective and expected to be used in various applications such as process monitoring, quality control, and fire defense.